1. Field of the Invention
The present invention relates to a sirocco fan, and more particularly, to a cutoff which divides an inlet and outlet side of the sirocco fan.
2. Discussion of the Related Art
Generally, a sirocco fan is kind of blower which is used for air-conditioning and ventilating, and is used in an air-conditioner or fan-coil heater. Also, the sirocco fan is called a multi-blade fan because it has about 30 to 50 blades. In this respect, the sirocco fan is compared differently with the radial fan having blades smaller than those of the sirocco fan.
FIGS. 1 and 2 are perspective and side views illustrating a related art sirocco fan.
The related art sirocco fan will be described with reference to FIGS. 1 and 2.
The sirocco fan 1 includes a scroll 2 which acts as air passage, and an impeller 3 which is provided in the scroll 2 and used for inlet and outlet of the air by the driving force of a motor (not shown).
The impeller 3 includes a pair of rims 3a and a plurality of blades 10 which are parallel to the rims 3a.
The scroll 2 is divided into a circular main body 2a where the impeller 3 is located and a diffuser 20 connected with the main body 2a. And the main body 2a includes an inlet wall 8 having an inlet 4 and a closed endwall 9 which is opposed to the inlet wall 8. A front end of the diffuser 20 acts as an outlet 5.
Therefore, in the sirocco fan 1, the air is sucked from the inlet 4 at the inlet wall 8 of the scroll 2 and exhausted out through the outlet 5 of the diffuser 20.
A reference numeral 6 which is not described is a bell-mouth which is located at the inlet 4 and fitted into the scroll 2 to guide sucking air and at the same time to shield the impeller 3.
In addition, the starting point in the diffuser 20 of the scroll 2 is a cutoff 7. The cutoff 7 is the standard point of the expansion radius of the scroll 2 and the important part to divide the inlet and outlet side.
In other words, since the cutoff 7 is the starting point which changes dynamic pressure into static pressure, it acts as the starting point of air outlet and quick change of pressure periodically occurs therein.
As shown in FIG. 2b, in the related art sirocco fan 1, the cutoff 7 is parallel to the blade 10 of the impeller 3 or it is tapered at about 2.degree. or less so as to easily separate the scroll 2 from the mold when producing the scroll 2.
Referring to FIGS. 2a and 2b, the operation of the related art sirocco fan will be described below.
Once the impeller 3 which is located in the scroll 2 is rotated by the driving force of the motor, suction power occurs by the rotation of the impeller 3, so that the air is sucked into the scroll 2 through the inlet 4 formed in the inlet wall 8 of the scroll 2. The sucked air is exhausted out of the scroll 2 through the outlet 5 formed in the scroll 2 while flowing inside the scroll 2 by the continuous rotation of the impeller 3.
However, the related art sirocco fan has the following problems.
First, since the cutoff 7 is parallel to the blade 10 of the impeller, abnormal noise occurs significantly due to the blade passing frequency (BPF). The reason is, when the blade 10 of the impeller 3 meets the cutoff 7 by the rotation of the impeller 3, the blade 10 is parallel to the cutoff 7 and at the same time the entire sections of the blade 10 meet the entire sections of the cuttoff 7 so that there occurs abnormal noise due to the BPF in the entire length direction of the blade.
Second, as shown in the FIG. 3, the speed of the incoming air does not decrease until it hits the endwall 9 of the sirocco fan 1. Therefore the incoming air retains the axial speed component until the time it passes through the blade 10 of the impeller 3. For this reason, the air that has hit the endwall 9 is re-circulated along the side of the scroll 2.
Also, since the scroll 2 cannot transfer the dynamic pressure into static pressure, the flow speed of the point close to the endwall 9 at the outlet 9 is comparatively fast and at the inlet wall 8 the flow speed is rather slow, thereby causing flow of the exhausting air to be irregular. Moreover, counterflow A may sometimes occur.
In summary, in the related art sirocco far, there is a problem that the air flow is poor due to abnormal noise.